The present invention relates to braking gears utilized in various branches of industry. The invention is of particular advantage for use in the cement, construction, and metallurgical industries for braking high-torque mechanisms such as mills of various types, conveyers, elevators, drilling rigs, and other similar apparatus.
The invention will now be described by way of example as used for braking tumbling mills well known in the cement industry, however, it should be understood that its application is not limited to this specific example.
At present in the cement industry there are used tumbling mills with a drive in which a low-speed electric motor co-axial with the mill is directly connected to the latter to impart rotary motion thereto. This motor provides for two operating conditions namely working fast rotation and slow rotation used in repairs. Due to a frequency converter, a smooth change in the speed of the motor, and hence, of the mill, is assured from zero to the maximum speed, say 20 rpm. A reliable braking gear is required to fix the mill in position, primarily during repair operations such as replacement of lining, tightening up of the fastenings, as well as in erecting the mill and unloading the grinding elements.
Known in the art is a braking gear for a tumbling mill operating directly from a co-axial low-speed electric motor and providing for stopping and fixing the mill in any position, for example, for tightening up the bolts and for other maintenance work.
This braking gear comprises a tightening beam with a brake shoe and two hydraulic cylinders which press the beam with the shoe to the brake member, e.g. the cylindrical portion of the mill body from below.
In this braking gear unavoidable oil leakage past the seals of the hydraulic cylinders, as well as possible damage to the pipelines, may cause drop of pressure in the hydraulic cylinders, thus releasing the brake which is not permissible. In view of the fact that in tumbling mills the bearing inserts are arranged only in the bottom part, the mill in its upper part is not checked against lifting under the pressure of the hydraulic cylinders from below, hence the brake shoe tightening effort cannot exceed the mass of that portion of the mill where the braking gear is installed. If the torque of the unbalanced mass of the mill exceeds the braking torque which can be obtained by pressing the shoes to the brake member of the mill, the latter will be rotated arbitrarily which is inadmissible for safety considerations. Since the hydraulic cylinders are mounted for a direct action on the brake shoes, the hydraulic cylinders and the seals are of large dimensions.
Also known in the art is braking gear comprising two brake shoes arranged symmetrically with respect to the member to be braked, such as a disk, pivotally joined with the levers hinged to a base, a resilient means interconnecting the levers, and a mechanism for turning the levers in bringing the brake shoes to and withdrawing from the member to be braked. The levers are mounted on the base so that they have movable ends of which like ones have on one side from the pivot pin pivotally mounted brake shoes with friction linings for interaction with both sides of the disk, while on the other side, i.e. on the reverse ends there is a resilient means incorporating a tension spring interconnecting the levers, and a mechanism represented as a single-acting hydraulic cylinder used for turning the levers in bringing the brake shoes to and withdrawing thereof from the brake disk.
In the initial position under the action of the resilient means the braking gear is always open, i.e. the brake shoes are withdrawn from the disk. When pressure fluid is applied to the hydraulic cylinder, it overcomes the pressure of the resilient means and the shoes grip the disk, thus braking it.
When pressure is released in the hydraulic cylinder, the resilient means moves the shoes away from the disk, thus releasing the latter.
The aforesaid braking gear is generally designed as normally open, though those skilled in the art can easily convert it into normally closed. However both designs cannot be efficiently used for safety reasons. Thus, the use of the prior art braking gear in the normally open version for fixing tumbling mills and similar devices during maintenance work involves the danger of brake release in case of a drop of pressure in the hydraulic cylinder.
In case where the prior art braking gear is used in the normally closed version, an accidental drop of pressure in the hydraulic cylinder can occur causing a stoppage of the processing line which adversely effects both the productivity thereof and quality of the product.
Besides, the use of the prior art braking gear for braking high-torque mechanisms requires great efforts to be developed by the resilient means and lever turning mechanism at the expense of increased size and mass of the resilient means and lever turning mechanism which complicates their manufacture, maintenance and increases cost.
Thus, the principal disadvantage of the prior art braking gear lies in that the mechanical connections of its members do not ensure reliable fixing both in the open and closed positions, i.e. in positions when the brake shoes are pressed to the member to be braked or withdrawn therefrom.